Lithographic offset printing plate on the market include presensitized plates (hereinafter referred to as PS plates) using a positively working sensitive agent mainly comprising a diazo compound and a phenolic resin or a negatively working sensitive agent mainly comprising an acrylic monomer or prepolymer. Since all of these printing plate exhibit low sensitivity, plate making is performed by exposing the precursor to light through an intimately contacted silver salt photographic film having previously formed thereon an original image. On the other hand, with the development of image processing by computers, storage of vast data, and data telecommunication techniques, an electronic compilation system has been put into practical use, in which processing including data input, correction, composing, assignment, and page composing are consistently operated by a computer and the data can be instantaneously output from a terminal plotter at a remote place by means of a rapid communication network or a satellite communication network. In the field of newspaper printing requiring promptness, there is a particularly high demand for the electronic compilation system. Also in the field where an original is stored in the form of a film and a printing plate is reproduced from the film when necessary, the development of ultra-high volume recording media such as photo discs is believed to enable storage of the originals as digital data in these recording media.
However, rarely has a direct type printing plate precursor which can provide a printing plate directly from the output of the terminal plotter been put into practical use. Under the present situation, the output from an electronic compilation system is transformed once into an image on a silver salt photographic film, which is then brought into contact with a PS plate for exposure to make a printing plate. This is because of difficulty in developing a direct type printing plate having a sufficiently high sensitivity to produce a printing plate within a practical amount of time by means of the light source used in the output plotter, e.g., an He-Ne laser, a semi-conductor laser, etc.
Electrophotographic photoreceptors are candidates for a photoreceptor having such a high photosensitivity that may provide a direct type printing plate. Known printing plate precursors utilizing electrophotography include offset printing plate materials comprising a zinc oxide-resin dispersion system as disclosed in JP-B-47-47610, 48-40002, 48-18325, 51-15766, and 51-25761 (the term "JP-B" as used herein means an "examined published Japanese patent application"). A printing plate of this type is subjected to electrophotographic processing to form a toner image and then treated with an oil-desensitizing solution (e.g., an acidic aqueous solution containing a ferrocyanide or a ferricyanide) to make the non-image area oil-insensitive in order to produce an offset printing plate. The printing plate thus obtained has a printing durability of from about 5000 to 10,000 prints and does not withstand further printing. Besides, a photoreceptor having a composition suitable for oil-desensitization would exhibit deteriorated electrostatic characteristics and deteriorated image quality. There is a further disadvantage in that a harmful cyan compound is employed in the oil-desensitizing solution.
JP-B-37-17162, 38-7758, 46-39405, and 52-2437 disclose a printing plate material comprising an organic photoconductive substance-resin system, in which a photoconductive insulating layer comprising an oxazole or an oxazole compound dispersed in a styrene-maleic anhydride copolymer is provided on a grained aluminum plate to construct an electrophotographic photoreceptor, which is electrophotographically processed to form a toner image and then treated with an alkaline organic solvent to dissolve out the non-image area to produce a printing plate.
The inventors previously proposed an electrophotographic light-sensitive printing plate material containing a hydrazone compound and barbituric acid or thiobarbituric acid as disclosed in JP-A-147656 (corresponding to U.S. Pat. No. 4,500,622; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Further, JP-A-147335, 59-152456, 59-168462, and 58-145495 disclose electrophotographic printing plate sensitized with dyes. However, all of these proposals turned out to fail to attain sufficient sensitivity to an oscillation wavelength region of an He-Ne laser or a semi-conductor laser that is of low cost and also contributes to reduction of the size of the apparatus.
There are also known non-uniform electrophotographic printing plates in which organic pigment particles are dispersed in a binder as an electron generating agent. For example, JP-A-55-161250, 56-146145, and 60-17751 disclose those in which a phthlocyanine pigment, an azo pigment or a condensed polycyclic quinone pigment, etc. is dispersed in a binder as an electronic charge generating agent. These non-uniform electrophotographic printing plates generally exhibit higher sensitivity as compared with uniform ones, but not even a few of them exhibit sufficient sensitivity for recording with an He-Ne laser. In some cases, however, they suffer from insufficiency in etching of the non-image area with an alkaline etching solution after toner image formation particularly when in using a grained and anodically oxidized aluminum plate. This is because the pigment particles dispersed in the binder are liable to adhere to the surface of the aluminum plate and remain there after etching. The lipophilic organic pigment remaining on the hydrophilic non-image area which is exposed by etching and is meant to exhibit ink-repellency naturally cause background stains on printing. The background stain is apt to be formed particularly when printing is carried out using a low-tack ink or under a high temperature condition or a condition using a reduced amount of dampening water.
Further, since the production of the pigment-dispersed non-uniform printing plate involves a step of pigment dispersion, the resulting photoreceptors show a wide scatter in performance, making it difficult to stably obtain electrophotographic printing plate having equal performance properties for the reasons set forth below. The electrophotographic characteristics of the photoreceptor are subject to variation due to non-uniformity in particle size and particle size distribution of the pigment. The viscosity of a pigment dispersion undergoes a drastic change in a short time after preparation, resulting in a large variation in the coating thickness. Therefore, the stability of the coating is so poor that difficulty arises in control of the thickness of the photosensitive layer, and electrophotographic characteristics such as charging properties are not constant.
As discussed above, the pigment-dispersed system non-uniform electrophotographic printing plates, though high in sensitivity, are liable to cause background stains, exhibit poor quality stability, and encounter difficulty in performing stable production, thus requiring a special device for production. Accordingly, a great demand exists to develop a uniform electrophotographic printing plate exhibiting high sensitivity without using a pigment.
When an electrophotographic photoreceptor is used as a printing plate, an alkali-soluble resin binder is usually used in order that the non-image area may be etched to expose the hydrophilic area. The alkali-soluble resins are generally inferior in compatibility with organic photoconductive compounds as compared with polycarbonate resins which are widely employed as binders for electrophotographic printing plates. Therefore, the amount of the organic photoconductive compound to be incorporated into an electrophotographic light-sensitive layer should be limited. The organic photoconductive compound, if incorporated in an amount exceeding a certain limit, would be precipitated from the light-sensitive layer to deteriorate electrophotographic performance properties. On the other hand, reduction of the amount of the organic photoconductive compound in the light-sensitive layer results in reduction of electrophotosensitivity. Hence, it has been difficult to increase electrophotosensitivity of the uniform electrophotographic printing plates sensitized with sensitizing dyes.